1. Field of the Invention
The present-invention relates to a GPS receiver, a position-detecting system and a positioning method, which are used for determining a current position from received GPS data.
2. Description of the Related Art
When in a power-on state, a conventional GPS receiver carries out processing to pick up a signal from or seizes one of a plurality of GPS satellites revolving around the earth when there is neither valid almanac nor ephemeris data. The total number of fully operating GPS satellites is 24. For example, the GPS receiver seizes a first GPS satellite.
If the GPS receiver successfully seizes the first GPS satellite, the receiver receives GPS data from the satellite and stores an almanac included in the data.
The almanac comprises information schematically showing the loci of all the GPS satellites. By referring to the almanac, it is possible to determine the orientation of each of the GPS satellites in each time zone. The almanac data is transmitted in 1/25 units at intervals of 30 seconds. Thus, the transmission of all the almanac data is completed in 12.5 minutes.
If the GPS receiver fails to seize the first GPS satellite, the GPS receiver proceed, for example, to seize the second GPS satellite.
If the GPS receiver successfully seizes the second GPS satellite, the receiver receives GPS data from the satellite and stores an almanac included in the GPS data.
If the GPS receiver fails to seize the second GPS satellite, the GPS receiver proceed, for example, to seize the third GPS satellite. The GPS receiver repeats the process of seizure until it successfully seizes the GPS satellite so as to acquire the almanac.
The GPS receiver stores the almanac thus acquired. It is assumed herein that the GPS receiver seizes the first GPS satellite successfully. The GPS receiver refers to the almanac included in the GPS data received from the first GPS satellite to learn the positions of the GPS satellites at the present point of time, that is, the positions of the GPS satellites recorded in the acquired almanac at the present point of time, in order to search the almanac for GPS satellites that are seizable (GPS satellites other than the first GPS satellite). For example, the seizable GPS satellite may be a satellite visible to the GPS receiver. It should be noted that, since a position or an altitude cannot be determined from the GPS data received from only one satellite, the GPS receiver seizes a plurality of GPS satellites.
When a seizable GPS satellite is identified from the search, the GPS receiver proceed to seize the identified GPS satellite by using a spread code corresponding to the satellite.
When the GPS receiver successfully seizes the GPS satellites, the receiver receives the GPS data from the satellites to find a difference between a time according to each satellite and a time according to the receiver. The time according to a GPS satellite is included in GPS data of the satellite. The GPS receiver then computes a distance from the receiver to each GPS satellite from the difference in time between the receiver and the satellite.
Then, the GPS receiver calculates the current position thereof from the differences in time between the receiver and the GPS satellites.
It should be noted that, if the precision of the computed current position is equal to or smaller than a first threshold value, a 2-dimensional positioning with respect to a latitude and a longitude is assumed to have been completed whereas, if the precision of the computed present position is equal to or smaller than a second threshold value, a 3-dimensional positioning with respect to a latitude, a longitude and an altitude is assumed to have been completed. The precision of the current position can be found geometrically from the layout of the GPS satellites. The first threshold value is greater than the second threshold value and, the smaller the threshold values, the higher the positional precision.
It should be noted that, if valid almanac data is found at a power-on time, the data is searched for seizable GPS satellites by using the time as a basis.
According to the construction of the conventional GPS receiver as described above, an almanac is searched for seizable GPS satellites so that the GPS satellites identified as a result of the search are seized. The current position is determined from the GPS data of the seized GPS satellites. The positioning operation is continued until the 2-dimensional positioning or the 3-dimensional positioning is completed even if a predetermined period of time has lapsed. However, there is a problem with respect to the length of time taken to finish the 2-dimensional positioning or the 3-dimensional positioning.
Accordingly, an object of the present invention is to provide a GPS receiver, a position-detecting system and a positioning method, which are capable of determining a current position in a short period of time by completing seizure of the GPS satellites required for the determination of the current position in a short period of time.
The GPS receiver according to the invention, causes a seizing means to cancel all currently seized satellites and to resume attempts to seize satellites by referring to an almanac stored in a storage means when the positioning means fails to complete a process of determining a current position within a predetermined period of time.
According to this aspect, it is possible to determine the current position efficiently.
The GPS receiver according to the invention may search for seizable satellites by referring to the almanac to identify current positions of the satellites.
According to this aspect, it is possible to seize the satellites within a short period of time.
The position-detecting system according to the invention is provided with a GPS receiver causes a seizing means to cancel all currently seized satellites and to resume attempts to seize satellites by referring to an almanac stored in a storage means when the positioning means fails to complete a process of determining a current position within a predetermined period of time.
According to this aspect, it is possible to seize a satellites necessary for positioning within a short period of time so that the current position is efficiently determined and presented.
The position-detecting system according to the invention may be provided with a power supply closing means for turning on a power supply of the GPS receiver when a predetermined operation is carried out before a power supply of the presenting means is turned on.
According to this aspect, the current position is immediately presented to a user as the user gets on a vehicle and turns on a power of the presenting means by turning on a switch for activating accessories to the vehicle.
The position-detecting system according to the invention may be constructed such that the power supply of the GPS receiver is turned on when a door-lock state of vehicle is released.
According to this aspect, the current position is immediately presented to a user as the user gets on a vehicle and turns on a power of the presenting means by turning on a switch for activating accessories to the vehicle.
The position-detecting system according to the invention may be constructed such that the power supply of the GPS receiver is turned on when a door of the vehicle is opened.
According to this aspect, the current position is immediately presented to a user as the user gets on a vehicle and turns on a power of the presenting means by turning on a switch for activating accessories to the vehicle.
The positioning method according to the invention cancels all currently seized satellites and resumes attempts to seize satellites by referring to a stored almanac when a process of determining a current position is not completed within a predetermined period of time.
According to this aspect, it is possible to determine the current position efficiently.
The positioning method according to the invention may search for seizable satellites by referring to the almanac to identify current positions of the satellites.
According to this aspect, it is possible to seize the satellites within a short period of time.